On a computer system display ergonomic principals often dictate that there be more gradations of displayable colors or intensities than what was originally provided for by the designers of the display panel component. Ergonomics also calls for less flicker from the display.
In some displays, producing more colors generates more flicker. The flicker in question occurs when a display controller utilizes certain techniques to increase the number of shades of intensity which a pixel on a LCD may display beyond the native shades originally provided for in the display. For example, a frame rate cycling ("FRC") technique has been employed with multishade displays which varies pel intensities over a sequence of frames in order to achieve a display having an intensity which is the average of the intensities being driven. In general, however, this technique produces flicker which must be minimized in order to provide an ergonomic display suitable for human interface.
Pixels on current raster color displays are composed of triplets of monochrome sub-pixels of primary or secondary colors. See display 101 in FIG. 1. See display 101, in FIG. 1A where, for each pixel location, only one out of three "sub-pixels" is shown, it being understood that in an actual color display three "subpixels" may be provided for each pixel. Each of these "sub-pixels" will be referred to herein as "pels", for example pel 102, FIG. 1A. Pels are the fundamental components of colors in a display. For purposes of the present application, the discussion will be in terms of pels so that a color panel can be viewed as a monochrome panel from the electrical standpoint.
U.S. Pat. No. 4,921,334, to Akodes, discloses a technique of frame rate cycling, herein referred to as Akodes' frame rate cycling approach, which increases the number of available shades from the native set of shades. When Akodes' approach is used, a pel's intrinsic shade capability may be nearly doubled. In accordance with Akodes' frame rate cycling approach, a pel is alternately driven with two intensities, across multiple frame refreshes, to produce a visual effect of the average (inter-shade) intensity. For example, in FIG. 1A, pel 102 is shown being driven with intensity levels 5 and 6 for an average intensity level of 5.5. This nearly doubles the number of available shades (less one due to the Nth intrinsic shade not having an N+1 shade with which to undergo FRC on a display with N intrinsic shades).
FIGS. 1A and 1B, illustrate Akodes' frame rate cycling approach, in which a FRC circuit alternately drives each pel undergoing FRC with a native intensity and an increased intensity from frame to frame. Pels 105 and 106, which are undergoing FRC, achieve average intensities over the sequence of frame refresh times of 2.5 and 1.5 respectively. However, as can be seen in FIGS. 1A and 1B, pels 105 and 106 cycle with their increased intensities in-phase with each other. Thus, from frame 1 to frame 2 both pels 105 and 106 are driven from a lower intensity to a higher intensity, i.e. in-phase with each other. The visual effect is that the increased intensities reinforce each other, accentuating flicker.
Another drawback with Akodes' frame rate cycling approach is that the number of shades produced is only about twice the number of native shades.
The in-phase FRC may be tolerable on displays in which there is a sufficient persistence on the pels so that the observer does not perceive a cycling of the intensities of the aggregate of FRC pels across the display. However, with some more recent display panels, persistence time has been reduced to yield faster response times. The use of Akodes' frame rate cycling approach on these displays is less desirable because the observer will perceive accentuated flicker. Because human perception is sensitive to aggregate features in displays, the observer perceives that the display's intensities are cycling.
Passive matrix liquid crystal displays differ from active matrix displays in that passive displays have longer persistence and generally have only a single bit per pel to control the intensity of the pel. In co-pending U.S. application Ser. No. 07/335,622, filed Apr. 10, 1989, now U.S. Pat. No. 5,185,602 entitled "Method and Apparatus for Producing Perception of High Quality Grayscale Shading on Digitally Commanded Displays" (hereinafter "the '622 Application"), and assigned to the same assignee as the subject application, there is discussed the reduction of perceivable flicker on an aggregate of pels in a passive display which are undergoing frame rate cycling by spreading the phases of the frame rate cycling in a specific pattern across the display.
One drawback of the system discussed in the '622 Application is that it does not support displays that have more than one bit per pel.
U.S. Pat. No. 4,769,713, issued Sep. 6, 1988, to Yasui, discusses the use of n-1 frames to generate n shades using frame rate cycling on panels which use a single command bit per pel. However, the disclosed circuit also does not support displays that have more than one command signal input per pel.